This invention relates generally to golf balls, and more specifically, to a wound golf ball having at least a center, a cover and a wound layer disposed therebetween having at least one thread. The cover can be formed of a polymer blend including a polyurethane composition and the core can include a polybutadiene composition. The golf balls of the present invention can provide improved velocity and a high resilience for low swing speed players to increase flight distance.
Conventional golf balls can be divided into two general groups: solid balls or wound balls. The difference in play characteristics resulting from these different types of construction can be quite significant. Balls having a solid construction are popular with golfers because they provide a very durable ball while also providing maximum distance. Solid balls are generally made with a solid core, usually made of a cross linked rubber, enclosed by a cover material. Typically the solid core is made of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents. In addition to one-piece solid cores, solid cores may also contain a number of outer layers, such as in a dual core golf ball. The cover is generally an ionomeric material, such as SURLYN(copyright), which is a tradename for a family of ionomer resins produced by E.I. DuPont de Nemours and Co. of Wilmington, Del. Covers are typically a single layer but may also include one or more layers, such as in a double cover having an inner and outer cover layer.
The combination of the solid core and ionomeric cover materials provide a ball that is very durable and abrasion resistant. Further, such a combination tends to impart a high initial velocity to the ball, which results in increased distance. Because these materials are very rigid, however, solid balls can have a hard xe2x80x9cfeelxe2x80x9d when struck with a club. Likewise, due to their construction, these balls tend to have a relatively low spin rate, which can provide greater distance and increases accuracy off the tee.
At the present time, wound golf balls are preferred by some golfers for the spin and feel characteristics afforded by such a construction. Wound balls typically have either a spherical solid rubber or liquid center, around which many yards of a tensioned elastomeric thread are wound. The wound core is then covered with a durable cover material, such as SURLYN(copyright) or similar material, or a softer cover material, such as balata or polyurethane. Wound balls are generally softer and provide more spin, which enable a skilled golfer to have more control over the ball""s flight and landing position. Particularly, with approach shots onto the green, the high spin rate of soft, wound balls enable the golfer to stop the ball very near its landing position.
To make wound golf balls, manufacturers use winding machines to stretch the elastic threads to various degrees of elongation during the winding process without subjecting the threads to unnecessary incidents of breakage. Generally, as the elongation and the winding tension increases, the compression and initial velocity of the ball increases. Thus, a more resilient wound ball is produced, which is desirable.
For wound golf balls, the thread is typically formed by a calendar and slitting method rather than an extrusion method. The calendared thread typically has a rectangular cross-section, while extruded thread generally has a circular cross-section.
A number of different windings have been disclosed for use in golf balls. U.S. Pat. No. 4,473,229 to Kloppenburg et al. discloses a golf ball having a core formed of graphite fibers and windings made of graphite filaments and resins. Yarns are made with the graphite filaments and resins, and as many as four or more yarns are combined to form a final yarn used for winding. U.S. Pat. No. 5,713,801 to Aoyama discloses use of a layer of high tensile elastic modulus fibers wound about the core. The fibers have a tensile elastic modulus of at least 10,000 ksi (10,000,000 psi). Also, U.S. Pat. No. 5,816,939 to Hamada et al. discloses a rubber thread for winding with a tensile strength retention of at least 70%, a hysteresis loss of no more than 50%, and an elongation of 900 to 1400%.
Prior art wound golf balls and cores typically use polyisoprene rubber thread wound onto the cores at elongations of between 500 to 1000%. The amount of thread required for a golf ball core is dependent on the elastic modulus of the thread in the elongated state. Elongated polyisoprene thread generally has an elastic modulus of 10,000 psi to 20,000 psi. Further, the properties, in particular resilience, of the wound ball or core are dependent on how well the thread packs during winding. The dimensions of the thread and winding pattern control the packing density. Present art polyisoprene threads are typically at least {fraction (1/16)} inches wide by 0.02 inches thick, measured prior to winding. Present art polyisoprene thread, however, is commonly produced in thicknesses between 0.014 inches and 0.024.
U.S. Pat. No. 6,149,535 discloses a thread for winding having at least about 10 individual strands that are each at most about 0.01 inches in diameter. Preferably, the thread has more than 25 strands with diameters of less than about 0.002 inches. The smaller thread dimension allows the thread to be wound more densely. Preferably, the elastic modulus of the thread is greater than 20 ksi when wound about a center. Preferably, the maximum elongation of the thread is greater than about 8%.
A variety of golf balls have been designed by manufacturers to provide a wide range of playing characteristics, such as compression, velocity, xe2x80x9cfeel,xe2x80x9d and spin. In addition to ionomers, one of the most common polymers employed is polybutadiene and, more specifically, polybutadiene having a high cis-isomer concentration.
The use of a polybutadiene having a high cis-concentration results in a very resilient and rigid golf ball, especially when coupled with a hard cover material. These highly resilient golf balls have a relatively hard xe2x80x9cfeelxe2x80x9d when struck by a club. Soft xe2x80x9cfeelxe2x80x9d golf balls constructed with a high cis-polybutadiene may also be constructed, however, they tend to have low resilience. In an effort to provide improved golf balls, various other polybutadiene formulations have been prepared, as discussed below.
U.S. Pat. No. 3,239,228 discloses a solid golf ball having a core molded of polybutadiene rubber with a high sulfur content, and a cover. The polybutadiene content of the core is stereo-controlled to the configuration 25-100 percent cis- and 0-65 percent trans-1,4-polybutadiene, with any remainder having a vinyl configuration of polybutadiene. A preferred embodiment of the polybutadiene golf ball core contains 35 percent cis-, 52 percent trans-, and 13 percent vinyl-polybutadiene. The level of trans- and vinyl-content are disclosed to be unimportant to the overall playing characteristics of the polymer blend.
British Patent No. 1,168,609 discloses a molding composition from which improved golf ball cores can be molded and which contains cis-polybutadiene as a basic polymer component. The core polymer component typically includes at least 60 percent cis-polybutadiene, with the remainder being either the trans- or vinyl-forms of polybutadiene. In a preferred embodiment, the core polybutadiene component contains 90 percent cis-configuration, with the remaining 10 percent being either the trans- or vinyl-configurations of 1,4-polybutadiene.
U.S. Pat. Nos. 3,572,721 and 3,572,722 disclose a solid, one- or two-piece golf ball, with the two-piece ball having a core and a cover. The cover material can include any one of a number of materials, or blends thereof, known to those of ordinary skill in the art, including trans-polybutadiene which may be present in an amount from at least 90 percent, with the remainder being the cis- and/or vinyl configuration.
British Pat. No. 1,209,032 discloses a two- or three-piece golf ball having a core and a cover. The core or cover material can be any material capable of being crosslinked. In particular, the material can be a polymer or a copolymer of butadiene or isoprene. Preferably, the polymer component is polybutadiene having a cis content of greater than 50 percent by weight.
U.S. Pat. No. 3,992,014 discloses a one-piece, solid golf ball. The golf ball material is typically polybutadiene, with a stereo-configuration selected to be at least 60 percent cis-polybutadiene, with the remaining 40 percent being the trans-polybutadiene and/or 1,2-polybutadiene (vinyl) isomers.
U.S. Pat. No. 4,692,497 discloses a golf ball and material thereof formed by curing a diene polymer including polybutadiene and a metal salt of an xcex1,xcex2-ethylenically unsaturated acid using at least two free radical initiators.
U.S. Pat. No. 4,931,376 discloses a process for producing butadiene polymers for use in various applications, including golf ball cover materials. One embodiment of the invention employs a blended polymeric resin material, including at least 30 percent by weight of a trans-polybutadiene polymer as a golf ball cover on a two-piece ball. In a preferred embodiment, the golf ball cover material contains a blend including 30 to 90 percent by weight of a trans-polybutadiene polymer.
U.S. Pat. No. 4,971,329 discloses a solid golf ball made from a polybutadiene admixture of cis-1,4 polybutadiene and 1,2 polybutadiene, a metal salt of an unsaturated carboxylic acid, an inorganic filler, and a free radical initiator. The admixture has about 99.5 percent to about 95 percent by weight of cis-1,4 polybutadiene and about 0.5 percent to about 5 percent by weight of 1,2 polybutadiene.
U.S. Pat. No. 5,252,652 discloses a one-piece or multi-layered golf ball core with improved flying performance from a rubber composition comprising a base rubber, preferably 1,4-polybutadiene with a cis-content of at least 40 mole percent, an unsaturated carboxylic acid metal salt, an organic peroxide, and an organic sulfur compound and/or a metal salt thereof. The organic sulfur compound and/or a metal salt is typically present in an amount from about 0.05 to 2 parts per hundred by weight and the organic peroxide is typically present in an amount from about 0.5 to 3 parts per hundred by weight of the total polymer component.
European Patent No. 0 577 058 discloses a golf ball containing a core and a cover that is formed as two separate layers. The inner layer of the cover is molded over the core and is formed from ionomer resin. The outer layer of the cover is molded over the inner layer and is formed from a blend of natural or synthetic balata and a crosslinkable elastomer, such as polybutadiene. In one embodiment of the outer layer of the cover, the elastomer is 1,4-polybutadiene having a cis-structure of at least 40 percent, with the remaining 60 percent being the trans-isomer. A preferred embodiment contains a cis-structure of at least 90 percent and more preferably, a cis-structure of at least 95 percent.
U.S. Pat. No. 5,421,580 discloses a wound golf ball having a liquid center contained in a center bag, a rubber thread layer formed on the liquid center, and a cover over the wound layer and liquid center. The cover material can include any one of a number of materials, or blends thereof, known to those of ordinary skill in the art, including trans-polybutadiene and/or 1,2-polybutadiene (vinyl), such that the cover has a JIS-C hardness of 70-85; preferred trans-percentages are not disclosed.
U.S. Pat. No. 5,697,856 discloses a solid golf ball having a core and a cover wherein the core is produced by vulcanizing a base rubber composition containing a butadiene rubber having a cis-polybutadiene structure content of not less than 90 percent before vulcanization. The amount of trans-polybutadiene structure present after vulcanization is 10 to 30 percent, as amounts over 30 percent are alleged to detrimentally result in cores that are too soft with deteriorated resilience performance, and to cause a decrease in golf ball performance. The core includes a vulcanizing agent, a filler, an organic peroxide, and an organosulfur compound.
British Patent No. 2,321,021 discloses a solid golf ball having a core and a cover formed on the core and having a two-layered cover construction having an inner cover layer and an outer cover layer. The outer cover layer is comprised of a rubber composite that contains 0.05 to 5 parts by weight of an organic sulfide compound. The core rubber composition comprises a base rubber, preferably 1,4-polybutadiene having a cis-content of at least 40 percent by weight, a crosslinking agent, a co-crosslinking agent, an organic sulfide, and a filler. The crosslinking agent is typically an organic peroxide present in an amount from 0.3 to 5.0 parts by weight and the co-crosslinking agent is typically a metal salt of an unsaturated fatty acid present in an amount from 10 to 40 parts by weight. The organic sulfide compound is typically present from 0.05 to 5 parts by weight.
U.S. Pat. No. 5,816,944 discloses a solid golf ball having a core and a cover wherein the core has a JIS-C hardness of 50 to 80 and the cover has a Shore-D hardness of 50 to 60. The core material includes vulcanized rubber, such as cis-polybutadiene, with a crosslinker, an organic peroxide, an organosulfur compound and/or a metal-containing organosulfur compound, and a filler.
Additionally, conventional polymers that have a high percentage of the trans-polybutadiene conformation, such as DIENE 35NF, from Firestone Corp., that has 40 percent cis-isomer and 50 percent trans-polybutadiene isomer, and mixtures of high-cis- and high-trans-polybutadiene isomers, such as CARIFLEX BR1220, from Shell Corporation, and FUREN 88, from Asahi Chemical Co., respectively, typically do not yield high resilience values and therefore are not desirable.
The most common polymers used by manufacturers to modify the properties of golf ball layers and/or covers have been ionomers, such as SURLYN, commercially available from E.I. DuPont de Nemours and Co., of Wilmington, Del. Recently, however, manufacturers have investigated the used of alternative polymers, such as polyurethane. For example, U.S. Pat. No. 3,147,324 is directed to a method of making a golf ball having a polyurethane cover.
Polyurethanes have been recognized as useful materials for golf ball covers since about 1960. Polyurethane compositions are the product of a reaction between a curing agent and a polyurethane prepolymer, which is itself a product formed by a reaction between a polyol and a diisocyanate. The curing agents used previously are typically diamines or glycols. A catalyst is often employed to promote the reaction between the curing agent and the polyurethane prepolymer.
Since 1960, various companies have investigated the usefulness of polyurethane as a golf ball cover material. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a diamine. U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent, and/or a difunctional glycol. The first commercially successful polyurethane covered golf ball was the Titleist(copyright) Professional(copyright) ball, first released in 1993.
Unlike SURLYN(copyright) or other ionomer-covered golf balls, polyurethane golf ball covers can be formulated to possess the softer xe2x80x9cfeelxe2x80x9d of balata covered golf balls. Conventional golf ball covers made from polyurethane, however, have not fully matched SURLYN(copyright)-covered golf balls with respect to resilience or the rebound that is in part a function of the initial velocity of a golf ball after impact with a golf club.
U.S. Pat. No. 3,989,568 discloses a three-component system employing either one or two polyurethane prepolymers and one or two polyols or fast-reacting diamine curing agents. The reactants chosen for the system must have different rates of reactions within two or more competing reactions.
U.S. Pat. No. 4,123,061 discloses a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a fast-reacting diamine curing agent.
U.S. Pat. No. 5,334,673 discloses a golf ball cover made from a composition of a polyurethane prepolymer and a slow-reacting polyamine curing agent and/or a difunctional glycol. Resultant golf balls are found to have improved shear resistance and cut resistance compared to covers made from balata or SURLYN(copyright).
U.S. Pat. No. 5,692,974 discloses methods of using cationic ionomers in golf ball cover compositions. Additionally, the patent relates to golf balls having covers and cores incorporating urethane ionomers. Improved resiliency and initial velocity are achieved by the addition of an alkylating agent such as t-butyl-chloride which induces ionic interactions in the polyurethane to produce cationic type ionomers.
International Patent Application WO 98/37929 discloses a composition for golf ball covers that comprises a blend of a diisocyanate/polyol prepolymer and a curing agent comprising a blend of a slow-reacting diamine and a fast-reacting diamine. Improved xe2x80x9cfeelxe2x80x9d, playability, and durability characteristics are exhibited.
It is thus desired to prepare wound golf balls having lower compression, i.e., a softer ball, while having the same or higher resilience than conventional wound balls. It is alternatively desired to obtain the same or lower compression while achieving greater resilience.
The present invention is directed to a wound golf ball with high resilience, such as for low swing speed players, to provide greater distance off the tee while conforming to the USGA golf rules. Methods for measuring the resiliency of golf balls are well known by those of ordinary skill in the art. One method of measuring the resiliency of a ball at impact is to utilize an air cannon or other means of propelling a ball at velocities equivalent to those of a golf club head. The balls are fired at a massive rigid block, with the inbound and outbound velocities being measured. The velocity may be measured by the use of light screens, which measure the time required for the ball to travel a fixed distance. The fixed distance divided by the transit time is equivalent to the average velocity of the ball over the fixed distance. The ratio of the outbound velocity to the inbound velocity is commonly referred to as the coefficient of restitution (xe2x80x9cCORxe2x80x9d). The COR is a direct measure of the resilience of a golf ball at a particular inbound velocity. Since golf balls behave in a linear-viscoelastic fashion, inbound ball velocity is functionally equivalent to club swing speed. In one embodiment, the present invention seeks to maximize the COR for low swing speed players. These players swing the club at the ball with low swing speeds, and thus tend to obtain lower ball velocity after impact and less distance off the tee.
The golf ball according to the present invention includes a center of one or more layers including a material formed from a polybutadiene material, or a reaction product thereof, having a molecular weight of greater than about 200,000 and a resilience index of at least about 40, a cover layer including a polyurethane composition, or reaction product thereof, formed from a prepolymer having no greater than 7.5 percent by weight unreacted isocyanate groups, and a wound layer including at least one thread material disposed between the center and the cover, each thread having at least one strand.
The invention further relates to a center, a wound layer surrounding the center, an inner cover layer surrounding the wound layer and having a first hardness, and an outer cover layer surrounding the inner cover layer and having a second hardness less than the first hardness and having a thickness of less than about 0.05 inches.
In one embodiment, the cover material has at least one of a dimple coverage of greater than about 60 percent, a hardness of the cover material from about 10 to 80 Shore D, or a flexural modulus of greater than about 500 psi, and wherein the golf ball has a compression from about 30 to 120. In one embodiment, the cover material hardness is from about 35 to 70 Shore D. In another embodiment, the resilience index is greater than about 50. Flexural modulus is measured by ASTM method D-6272-98. In yet another embodiment, the compression of the golf ball is from about 50 to 110.
In one embodiment, the cover includes an inner cover layer and an outer cover layer, the inner cover layer being disposed between the wound layer and the outer cover layer. In one embodiment, the inner cover layer is harder than the outer cover layer. In yet another embodiment, the inner cover layer includes at least one thermoplastic material.
In one embodiment, the polyurethane composition includes at least one isocyanate and at least one curing agent. In yet another embodiment, the polyurethane composition includes at least one isocyanate, at least one polyol, and at least one curing agent. In a preferred embodiment, the isocyanate includes 4,4xe2x80x2-diphenylmethane diisocyanate, polymeric 4,4xe2x80x2-diphenylmethane diisocyanate, carbodiimide-modified liquid 4,4xe2x80x2-diphenylmethane diisocyanate, 4,4xe2x80x2-dicyclohexylmethane diisocyanate, p-phenylene diisocyanate, toluene diisocyanate, isophoronediisocyanate, p-methylxylene diisocyanate, m-methylxylene diisocyanate, o-methylxylene diisocyanate, or a mixture thereof. In another preferred embodiment, the at least one polyol includes a polyether polyol, hydroxy-terminated polybutadiene, polyester polyol, polycaprolactone polyol, polycarbonate polyol, or mixtures thereof. In yet another preferred embodiment the curing agent includes a polyamine curing agent, a polyol curing agent, or a mixture thereof. In a more preferred embodiment, the curing agent includes a polyamine curing agent. In a most preferred embodiment, the polyamine curing agent includes 3,5-dimethylthio-2,4-toluenediamine, or an isomer thereof; 3,5-diethyltoluene-2,4-diamine, or an isomer thereof; 4,4xe2x80x2-bis-(sec-butylamino)-diphenylmethane; 1,4-bis-(sec-butylamino)-benzene, 4,4xe2x80x2-methylene-bis-(2-chloroaniline); 4,4xe2x80x2-methylene-bis-(3-chloro-2,6-diethylaniline); trimethylene glycol-di-p-aminobenzoate; polytetramethyleneoxide-di-p-aminobenzoate; N,Nxe2x80x2-dialkyldiamino diphenyl methane; p,pxe2x80x2-methylene dianiline; phenylenediamine; 4,4xe2x80x2-methylene-bis-(2-chloroaniline); 4,4xe2x80x2-methylene-bis-(2,6-diethylaniline); 4,4xe2x80x2-diamino-3,3xe2x80x2-diethyl-5,5xe2x80x2-dimethyl diphenylmethane; 2,2xe2x80x2,3,3xe2x80x2-tetrachloro diamino diphenylmethane; 4,4xe2x80x2-methylene-bis-(3-chloro-2,6-diethylaniline); or mixtures thereof.
In a different preferred embodiment, the curing agent includes a polyol curing agent. In a more preferred embodiment, the polyol curing agent includes ethylene glycol; diethylene glycol; polyethylene glycol; propylene glycol; polypropylene glycol; lower molecular weight polytetramethylene ether glycol; 1,3-bis(2-hydroxyethoxy)benzene; 1,3-bis-[2-(2-hydroxyethoxy)ethoxy]benzene; 1,3-bis-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}benzene; 1,4-butanediol; 1,5-pentanediol; 1,6-hexanediol; resorcinol-di-(xcex2-hydroxyethyl)ether; hydroquinone-di-(xcex2-hydroxyethyl)ether; trimethylol propane, or mixtures thereof.
In one embodiment, the prepolymer has from about 2.5 percent up to 7.5 percent by weight unreacted isocyanate groups. In another embodiment, the cover layer has a thickness of less than about 0.05 inches. In yet another embodiment, the center has a Mooney viscosity from about 40 to about 80. In a preferred embodiment, the Mooney viscosity is from about 45 to about 60.
In another embodiment, the polybutadiene material has a vinyl-isomer content of less than about 2 percent by weight. In another embodiment, and possibly in addition to the previous embodiment, the polybutadiene has a cis-isomer content of at least about 95 percent by weight.
In one embodiment, the center has an outer diameter of at least about 1.3 inches, preferably from about 1.3 inches to 1.55 inches. In one preferred embodiment, the center has an outer diameter of about 1.34 to 1.4 inches. In another embodiment, the center includes a material formed from a conversion reaction of polybutadiene having a first amount of trans-isomer, a free radical source, and at least one cis-to-trans catalyst. In a preferred embodiment, the reaction occurs at a temperature and for a time sufficient to form a polybutadiene reaction product having a second amount of trans-isomer greater than the first amount of trans-isomer. In one embodiment, the cis-to-trans catalyst includes at least one of an organosulfur compound, an inorganic sulfur compound, an aromatic organometallic compound, a metal-organosulfur compound, tellurium, selenium, elemental sulfur, a polymeric sulfur, or an aromatic organic compound. Preferably, the catalyst includes an organosulfur component, and in one preferred embodiment the catalyst includes at least one of 4,4xe2x80x2-diphenyl disulfide, 4,4xe2x80x2-ditolyl disulfide, or 2,2xe2x80x2-benzamido diphenyl disulfide, or a combination thereof. The cis-to-trans catalyst is typically present in an amount from about 0.1 to 10 parts per hundred of polybutadiene.
In one embodiment, which further includes an additional intermediate layer between the center and the cover layer, the intermediate layer includes a material formed from a conversion reaction of polybutadiene having a first amount of trans-polybutadiene, a free radical source, and a cis-to-trans catalyst includes at least one organosulfur component, wherein the intermediate layer has an outer diameter of at least about 1.58 inches, and wherein the center has an outer diameter of less than about 1.55 inches.
In another embodiment, the cover includes an inner cover layer and an outer cover layer, the inner cover layer being disposed between the wound layer and the outer cover layer. In a preferred embodiment, at least one of the inner and outer cover layer has a thickness of less than about 0.05 inches. In another preferred embodiment, the inner cover layer includes an ionomer resin, a polyurethane, a polyetherester, a polyetheramide, a polyester, a dynamically vulcanized elastomer, a polyurea, a functionalized styrenebutadiene elastomer, a metallocene polymer, nylon, acrylonitrile butadiene-styrene copolymer, or a blend thereof.
In one embodiment, the inner cover has an outer diameter of at least about 1.55 inches. In another embodiment, the inner cover has an outer diameter of about 1.58 to about 1.64 inches. In one embodiment, the polyurethane used in forming one or both cover layers is thermoplastic or thermoset.
In one embodiment for low swing speed players, the coefficient of restitution of the golf ball at a club head speed of 160 ft/s is at least about 0.76 and the magnitude of the gradient of the coefficient of restitution to an inbound velocity is at least about 0.001 s/ft.
The invention also relates to a golf ball having at least a center including a polybutadiene having a molecular weight of greater than about 300,000 and a resilience index of at least about 40, a wound layer surrounding the center, having an outer diameter of at least about 1.51 inches, and including at least one thread material disposed between the center and the cover, each thread having at least one strand, an inner cover layer surrounding the outer core layer, and an outer cover layer disposed around the inner cover layer, the outer cover layer including a polyurethane composition formed from a prepolymer having less than 7.5 percent by weight unreacted isocyanate groups.
The invention also relates to a center including a polybutadiene having a molecular weight of greater than about 300,000 and a resilience index of at least about 40; a hoop stress layer surrounding the center, having an outer diameter of at least about 1.51 inches, and disposed between the center and the cover, wherein the hoop stress layer includes a glass, polyamide, aromatic polyamide, carbon, or metal fiber having a tensile strength of at least about 250 kpsi and a modulus of at least about 10,000 kpsi; and a cover having at least one layer disposed around the wound hoop stress layer, the cover including a polyurethane composition formed from a prepolymer having less than 7.5 percent by weight unreacted isocyanate groups.